The goal of this project is to use quantitative light and electron microscopy along with in situ hybridization and immunocytochemistry to study cellular and molecular mechanisms of myelin formation, breakdown and regeneration. In nerve lesions involving injury to myelinated fibers, return of function depends on regeneration of transected axons and a series of interactions that they must have with Schwann cells before and during myelin regeneration. To study how regenerating axons might regulate proliferation of Schwann cells and their production of laminin, supernatants of proximal nerve segments which contain regenerating axons were prepared and added to cultured Schwann cells. Compared to those from normal nerves, supernatants 24 and 48 hours after transection increased Schwann cell proliferation significantly as measured by an ELISA for bromodeoxyuridine (BrdU) incorporation and by the numbers of anti-BrdU immunoreactive Schwann cell nuclei. The 24 hour supernatants also significantly increased laminin production, which was assessed by anti-laminin Schwann cell immunoreactivity before and after fixation and by ELISAs normalized for microculture protein amounts. In other tissue culture experiments, the fine structure of Schwann cell was studied after exposure to psychosine. Cytoplasmic inclusions and progressive changes in mitochondria and other organelles were found at low doses; higher doses produced rapid retraction of cell processes and necrosis. These findings add to the evidence suggesting that the accumulation of psychosine in myelin-forming cells is cytotoxic and may have an important role in producing the myelin breakdown seen in Krabbe's disease, an autosomal recessive neurological disorder of infancy. Finally, morphometric, immunocytochemical and in situ hybridization studies of six cases of Balo's concentric sclerosis showed that this rare clinical and pathological syndrome represents an acute progressive form of multiple sclerosis and that the main changes seen in partially myelinated areas are those found in ongoing myelin breakdown rather than myelin regeneration.